An internal combustion engine may be used as a motor vehicle drive unit. Within the context of the present disclosure, the expression “internal combustion engine” encompasses diesel engines and Otto-cycle engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and hybrid drives which comprise not only the internal combustion engine but also an electric machine which can be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
In recent years, there has been a trend in development toward small, highly supercharged engines, wherein supercharging is primarily a method of increasing power, in which the air required for the combustion process in the engine is compressed. The economic significance of said engines for the automotive engineering industry is ever increasing.
For supercharging, use is often made of an exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine and expands in the turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooler is commonly provided in the intake system downstream of the compressor, by means of which charge-air cooler the compressed charge air is cooled before it enters the at least one cylinder. The cooler lowers the temperature and thereby increases the density of the charge air, such that the charge-air cooler also contributes to improved charging of the cylinders, that is to say to a greater air mass. Compression by cooling takes place.
The advantage of an exhaust-gas turbocharger in relation to a mechanical charger is that no mechanical connection for transmitting power exists or is required between charger and internal combustion engine. While a mechanical charger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
As already mentioned, supercharging serves for increasing power. The air required for the combustion process is compressed, as a result of which a greater air mass can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable means for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and an improved power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower.
Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption, that is to say to improve the efficiency of the internal combustion engine.
It is a further basic aim to reduce pollutant emissions. Supercharging can likewise be expedient in solving this problem. With targeted configuration of the supercharging, it is possible specifically to obtain advantages with regard to efficiency and with regard to exhaust-gas emissions. To adhere to future limit values for pollutant emissions, however, further engine-internal measures are necessary in addition to the supercharging arrangement. For example, exhaust-gas recirculation serves for reducing the untreated nitrogen oxide emissions. Here, the exhaust-gas recirculation rate xEGR is determined as xEGR=mEGR/(mEGR+mfresh air), where mEGR denotes the mass of recirculated exhaust gas and mfresh air denotes the supplied fresh air.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase in all engine speed ranges. According to the prior art, a severe torque drop is however observed in the event of a certain engine speed being undershot.
Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. If the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. Consequently, toward lower engine speeds, the charge pressure ratio likewise decreases. This equates to a charge pressure drop or torque drop.
In practice, the relationships described above often lead to the use of a small exhaust-gas turbocharger, that is to say an exhaust-gas turbocharger with a small turbine cross section, whereby the turbine pressure ratio can be increased. This however impairs the supercharging at high engine speeds, and merely shifts the torque drop toward lower engine speeds. Furthermore, said approach, that is to say the reduction in size of the turbine cross section, is subject to limits because the desired supercharging and performance increase should be possible without restriction and to the desired extent even at high engine speeds.
In the prior art, it is sought, using a variety of measures, to improve the torque characteristic of a supercharged internal combustion engine.
It is sought to do this for example by means of a small design of the turbine cross section and simultaneous exhaust-gas blow-off, wherein the exhaust-gas blow off can be controlled by means of charge pressure or by means of exhaust-gas pressure. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a critical value, a part of the exhaust-gas flow is, within the course of the so-called exhaust-gas blow-off, conducted via a bypass line past the turbine. Said approach however—as already discussed above—has the disadvantage that the supercharging behavior is inadequate at relatively high engine speeds.
The torque characteristic of a supercharged internal combustion engine may furthermore be improved by means of multiple turbochargers arranged in parallel, that is to say by means of multiple turbines of relatively small turbine cross section arranged in parallel, wherein turbines are activated successively with increasing exhaust-gas flow rate.
The torque characteristic may also be advantageously influenced by means of multiple exhaust-gas turbochargers connected in series. By connecting two exhaust-gas turbochargers in series, of which one exhaust-gas turbocharger serves as a high-pressure stage and one exhaust-gas turbocharger serves as a low-pressure stage, the engine characteristic map can advantageously be expanded, specifically both in the direction of smaller compressor flows and also in the direction of larger compressor flows.
In particular, with the exhaust-gas turbocharger which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows, as a result of which high charge pressure ratios can be obtained even with small compressor flows, which considerably improves the torque characteristic in the lower engine speed range. This is achieved by designing the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by means of which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again upstream of the low-pressure turbine. In the bypass line there is arranged a shut-off element for controlling the exhaust-gas flow conducted past the high-pressure turbine.
Shifting the surge limit of the compressor of an exhaust-gas turbocharging arrangement further, or as far as possible, toward small compressor flows is also advantageous for other reasons.
In the case of small compressor flows, the speed of the charge-air flow relative to the intake system decreases to such an extent that the flow approaching the rotating impeller blades runs at an excessively large angle, and the charge-air flow detaches from the airfoil-like blades. The resulting pressure fluctuations on the blades lead to increased noise emissions, and possibly to damage of the blades. Further adverse effects that can arise are mass flow fluctuations and a severe decrease in efficiency.
This effect can be counteracted by means of a variable compressor geometry. By adjustment of the blades of a guide wheel provided upstream, it is possible for the flow approaching the rotating impeller blades, that is to say the approaching-flow angle, to be manipulated to a limited extent, whereby the surge limit of the compressor is shifted in the compressor characteristic map toward small compressor flows.
Equipping a compressor with a variable compressor geometry is however expensive. Furthermore, the capacity for manipulation by means of a variable compressor geometry is also subject to limits, as an adjustment of the guide wheel is possible only to a certain extent. Furthermore, in the case of relatively large compressor flows, a guide device constitutes a flow resistance, and is thus somewhat obstructive.
The inventors herein have recognized the above issues and provide a method to at least partly address them. In one example a method, comprises during a first condition, flowing exhaust gas from downstream of a turbine to upstream of a compressor via a tangential flow duct of an exhaust gas recirculation (EGR) injector circumferentially surrounding an intake passage upstream of the compressor, and during a second condition, flowing exhaust gas from downstream of the turbine to upstream of the compressor via a radial flow duct of the EGR injector.
In this way, EGR flow may be provided to upstream of a compressor via a radial flow and/or tangential flow duct of an injector that circumferentially surrounds the intake passage upstream of the compressor. The portion of EGR that flows through each respective flow duct may be controlled by a control valve. During conditions of compressor surge, for example, more EGR may be directed through the tangential flow duct in order to create swirl upstream of the compressor in a direction equal to the direction of compressor rotation. In doing so, compressor surge may be mitigated.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.